1. Field of the Invention
The present invention is a nonaqueous, heat-curable, two-component coating with an improved balance of scratch resistance and resistance to environmental damage, especially to etching caused by exposure to acid rain.
2. Discussion of the Background
Two-component polyurethane (PUR) coatings are used as top coats in the automotive industry because they have superior resistance to environmental damage, particularly etching caused by exposure to acid rain, compared to conventional coating systems comprising cross-linking melamine resins (W. Wieczorrek in: Stoye/Freitag, Lackharze, pp. 215 ff., C. Hanser Verlag, 1996; J. W. Holubka et al., J. Coat. Techn. Vol. 72, No. 901, p. 77, 2000). Generally, PUR coatings are composed of poly(meth)acrylate resins having OH functional groups and polyisocyanates based on hexamethylene diisocyanate (HDI). The good resistance of these PUR coatings to environmental damage can be significantly improved by substituting IPDI (isophorone diisocyanate) polyisocyanates for some of the HDI (WO 93/05090). However, such modified PUR top coating materials have poorer scratch resistance than pure HDI cross-linked polyisocyanate coatings (Industrie Lackierbetrieb, 61, p. 30, 1993).
Reaction products of polyisocyanates with secondary 3-aminopropyltrialkoxysilanes are known. For example, 3-aminopropyltrialkoxysilanes modified with esters of maleic or fumaric acid are reacted with isocyanate prepolymers in order to improve the adhesion of corresponding coating systems or sealing compounds and to reduce the detrimental evolution of CO2 (European Patent 596360, U.S. Pat. No. 6,005,047). Such isocyanate adducts are also described for the preparation of aqueous PUR dispersions (European Patent 924231) or as hardener components for aqueous two-component PUR systems (European Patent 872499, European Patent 949284). In the great majority of cases, the coatings are cured at ambient temperature or slightly elevated temperature in the presence of water vapor.
European Patent 549643, International Patent WO 92/11327, International Patent WO 92/11328 and U.S. Pat. No. 5,225,248 describe the use of silane group-containing resins in nonaqueous, heat-cured clear coatings in order to improve the damage resistance, especially with respect to acid rain, for top coatings of automobiles. In these patents, the clear coatings contain cross-linkers based on silane group-containing poly(meth)acrylate resins, on hydroxyl-group-containing poly(meth)acrylate resins, and on melamine resins. Such clear coatings are commonly considered to be acid-resistant, but are clearly inferior to two-component PUR coatings (J. W. Holubka et al., J. Coat. Techn. Vol. 72, No. 901, p. 77, 2000).
Because the quality requirements for top coats for automobiles have become increasingly stringent, an improved balance of scratch resistance and environmental damage resistance is desired. The present invention is a two-component coating which has a better balance of resistance to environmental damage, and at the same time greater resistance to mechanical damage, especially scratch resistance.
The present invention comprises a nonaqueous, heat-cured, two-component coating containing
A) a solvent-containing polyol component and
B) a cross-linking component, comprising a cross-linking agent prepared by reacting at least one aliphatic and/or cycloaliphatic polyisocyanate having 2 to 6 NCO functional groups, wherein 0.1 to 95 mol % of the originally free isocyanate groups of the polyisocyanate are reacted with N-alkyl-3-aminopropyltrialkoxysilanes and/or N-aryl-3-aminopropyltrialkoxysilanes, and the weight ratio of the polyol of component A to the cross-linking agent of component B in the coating is 6:1 to 1:2.
In principle, polyol component A may include all polyols containing more than two OH groups. For example, polyol component A may be (meth)acrylic copolymers containing hydroxyl groups, saturated polyester polyols, polycarbonate diols, polyether polyols, polyester-urethane polyols, or mixtures thereof.
The solvent used in component A may be any conventional organic solvent used in coatings. For example, the solvent may be a ketone (i.e., acetone, methyl ethyl ketone, etc.), an aliphatic or aromatic hydrocarbon, and ester (i.e., butyl actetate, etc.), or any other suitable solvent which can dissolve the polyol. However, the solvent should not include water.
The (meth)acrylic copolymers containing hydroxyl groups may be resins having the monomer composition described in, for example, International Patent WO 93/15849 (p. 8, line 25 to p. 10, line 5), or else in German Patent 19529124. For example, the (meth) acrylate copolymers may be copolymers containing (meth)acrylate esters and (meth)acrylate hydroxyalkylesters (e.g., 2-hydroxyethyl methacrylate and 2-hydroxypropyl methacrylate). The acid number of the (meth)acrylic copolymer may be adjusted by using (meth)acrylic acid as a comonomer, and should be 0 to 30 mg KOH/g, preferably 3 to 15 mg KOH/g. The number-average molecular weight (determined by gel permeation chromatography versus a polystyrene standard) of the (meth)acrylic copolymer is preferably 2,000 to 20,000 g/mol, and the glass transition temperature is preferably xe2x88x9240xc2x0 C. to +60xc2x0 C. The hydroxyl content of the (meth)acrylic copolymer of the present invention, which may be adjusted by adding hydroxyalkyl(meth)acrylate comonomers, is preferably 70 to 250 mg KOH/g, more preferably 90 to 190 mg KOH/g. Any other vinyl monomer which can be copolymerized with the (meth)acrylate monomers may also be used, provided these additional monomers do not compromise the desired performance properties of the ultimate coating (i.e., resistance to scratching and environmental damage). By xe2x80x9c(meth)acrylatexe2x80x9d, we mean monomers derived from methacrylic acid and/or acrylic acid (i.e., methacrylic esters and acrylic esters, methacrylic hydroxyalkyl esters and acrylic hydroxyalkyl esters, etc.).
Polyester polyols which may be used in the present invention include resins prepared by reacting dicarboxylic and polycarboxylic acid monomers with diols and polyols, as described in, for example, Stoye/Freitag, Lackharze, C. Hanser Verlag, 1996, p. 49, or in International Patent WO 93/14849. The polyester polyols may also be polyaddition products of caprolactone and low molecular weight diols and triols, such as those available under the name TONE (Union Carbide Corp.) or CAPA (Solvay/Interox). The theoretical number-average molecular weight of such polyester polyols is preferably 500 to 5,000 g/mol, more preferably 800 to 3,000 g/mol, and the average number of functional groups per molecule is 2.0 to 4.0, preferably 2.0 to 3.5.
Polyester-urethane polyols which may be used in the present invention include those described in European Patent 140186. For example, suitable polyester-urethane polyols may be prepared by reacting an organic polyisocyanate with any of the polyester polyols described above (i.e., polyester polyols formed by reacting an organic polycarboxylic acid with a polyol). Preferred polyester-urethane polyols include those prepared by reacting any of HDI, IPDI, trimethylhexamethylene diisocyanate (TMDI) or (H12-MDI) with a polyester polyol. The number-average molecular weight of such polyester-urethane polyols is preferably 500 to 2,000 g/mol, and the average number of functional groups per molecule is 2.0 to 3.5.
Cross-linking component B comprises a cross-linking agent prepared by reacting at least one aliphatic and/or cycloaliphatic polyisocyanate containing 2 to 6 NCO functional groups per molecule, wherein 0.1 to 95 mol % of the originally free isocyanate groups of the polyisocyanate are reacted with at least one N-alkyl-3-aminopropyltrialkoxysilane and/or at least one N-aryl-3-aminopropyltrialkoxysilane.
The polyisocyanate of component B may be any diisocyanate or a polyisocyanate based on hexamethylene diisocyanate (HDI), isophorone diisocyanate (IPDI), bis(4-isocyanatocyclohexyl)methane, (H12-MDI), tetramethylxylylene diisocyanate (TMXDI), 1,3-bis(isocyanatomethyl)cyclohexane (1,3-H-XDI), 2,2,4-trimethyl-1,6-diisocyanatohexane and 2,4,4-trimethyl-1,6-diisocyanatohexane (TMDI), 2-methylpentene diisocyanate-1,5 (MPDI), norbornyl diisocyanate (NBDI), lysine triisocyanate (LTI) or 4-isocyanatomethyl-1,8-octamethylene diisocyanate (NTI) or mixtures of these diisocyanates, and the mean number of NCO functional groups per molecule ranges from 2.0 to 6.0. Suitable diisocyanates include, but are not limited to HDI, IPDI, H12-MDI, TMXDI, 1,3-H-XDL TMDI, MPDI, NBDI, LTI, and NTI.
In order to have more than two NCO functional groups per isocyanate molecule, polyisocyanates are preferably usedxe2x80x94alone or in mixturesxe2x80x94such as those synthesized by trimerization, dimerization or formation of urethane, biuret or allophanate groups from any of the above diisocyanates, as well as mixtures thereof with monomers. Such polyisocyanates or polyisocyanate/monomer mixtures may be chain extended or branched if appropriate by reaction with difunctional or polyfunctional H-acid components such as diols or polyols and/or diamines or polyamines.
The aliphatic and/or cycloaliphatic cross-linking component B of the present invention is obtained by modifying the polyisocyanates by reaction with N-alkyl-3-aminopropyltrialkoxysilanes and/or N-aryl-3-aminopropyltrialkoxysilanes having the general formula I
Rxe2x80x94NHxe2x80x94(CH2)3xe2x80x94Si(OR1)3 xe2x80x83xe2x80x83(I)
where R is an alkyl, iso-alkyl, tert-alkyl, cycloalkyl or aryl group having 1 to 10 carbon atoms and the R1 groups may be, independently of one another, an alkyl or iso-alkyl group having 1 to 8 carbon atoms. Preferably, the compounds of formula I may be n-butyl-3-aminopropyl- triemthoxysilane, n-butyl-3-aminopropyltriethoxysilane, n-butyl-3-aminopropyl-tri-i- propoxysilane, methyl-3-aminopropyl-trimethoxysilane, methyl-3-aminopropyltriethoxy- silane, methyl-3-aminopropyltri-i-propoxysilane, phenyl-3-aminoproplytriethoxysilane, phenyl-3-aminopropyltrimethoxysilane, phenyl-3-aminopropyltri-i-propoxysilane, cyclohexyl-3-aminopropyltriethoxysilane, cyclohexyl-3-aminopropyltrimethoxysilane, or cyclohexyl-3-aminopropyltri-i-propoxysilane.
An alternative method for preparing cross-linking component B comprises the partial reaction of monomeric diisocyanates with the aforementioned compounds of formula I, followed by trimerization, dimerization to form a polyisocyanate, or by formation of urethane, biuret or allophanate compounds from the polyisocyanate, and then, if necessary, distilling away residual monomers. Mixtures of unmodified polyisocyanates and completely reacted polyisocyanates may also be used in the present invention.
The cross-linking component B of the present invention may be carried out in the liquid phase at temperatures below 130xc2x0 C., and any aprotic solvents which are typically used in PUR technology may be used, if necessary. In addition, catalysts and/or stabilizers may also be used, if necessary.
The nonaqueous, two-component coating of the present invention usually contains solvents well known in coating technology, such as ketones, esters or aromatics, and adjuvants such as stabilizers, UV stabilizers, hindered amine light stabilizers (HALS), catalysts, leveling agents or rheological adjuvants, such as xe2x80x9csag control agentsxe2x80x9d, microgels or pyrogenic silicon dioxide in typical concentrations.
Catalysts known to be effective in PUR coatings, such as organic Sn(IV), Sn(II), Zn and Bi compounds or tertiary amines (PUR catalysts), are preferred catalysts.
Latent sulfonic-acid-based catalysts, i.e., organic sulfonic acids neutralized by amines or covalent adducts of organic sulfonic acids with epoxy-containing compounds, particularly those described in German Unexamined Patent Application DE-OS 2356768, are also suitable catalysts.
Combinations of PUR catalysts and latent sulfonic-acid-based catalysts are particularly preferred. An especially preferred coating has 0.01 to 0.5 wt % PUR catalyst and 0.1 to 7 wt % latent sulfonic-acid-based catalyst, relative to the nonvolatile organic constituents of the coating.
If necessary inorganic or organic coloring and/or special effect pigments (i.e., metal flakes, etc.) which are commonly used in coating technology may also be incorporated into component A.
The weight ratio of the polyol of component A to component B in the coating of the present invention ranges from 6:1 to 1:2. Other nonvolatile organic components of the coating may include nonvolatile resins, catalysts, adjuvants, etc. in the coating. Volatile components include solvents which have an appreciable vapor pressure at room temperature and atmospheric pressure (e.g., ketone, ester or aromatic solvents typically used in coatings).
Components A and B are intimately mixed until a homogeneous solution is obtained, just before they are used in a coating process. So-called two-component mixing systems may be used for mixing components A and B on a large industrial scale.
The coating of the present invention may be applied by known methods such as spraying, dipping, rolling or doctoring. The substrate to be coated may already be coated with other coating layers. The coating of the present invention is particularly suitable for use as a clear coating, in which case it is applied in the so-called wet-in-wet process on one or more base coating layers, which are all cured at the same time. The coating of the present invention may be cured at a temperature range of 100 to 180xc2x0 C. The coating of the present invention may be used for production of clear or top coats for motor vehicles.